Paper 6 SCNZ - Powerpoint - AFC Connections - A designers View

SCNZ Steel Innovations Conference 2013
“Concentric Braced Frames with AFC
Connections – A Designers View”
Ben Leslie, Sean Gledhill & Masoud Moghaddasi

Presentation Overview
• Evolution towards Low Damage Design for CBF’s
• Asymmetric Friction Connection (AFC) Concept and
Configuration
• Key Design Considerations for AFC’s - Existing Theory
and Developments Between Industry and Research
• Key Benefits of AFC’s over other CBF Systems
• Conclusions and Summary

Evolution towards Low Damage Design for CBF’s
(a) Traditional Ductile CBF’s
• Ductility achieved through compression buckling of brace
- High damage to brace
- Difficult to predict overstrengths
- Limited resilience to large cycles of earthquakes/aftershocks
• Due to less favourable buckling response, NZS3404 requires:
- Braces in Pairs
- Maximum height limitations
- Increased seismic demand (Cs factor)
Sciencedirect.com
• Need for a system which:
- Protects brace from buckling
- Dissipates energy through another mechanism

Evolution towards Low Damage Design for CBF’s
(b) Buckling Restrained Braces (BRB’s)
• Steel insert in grouted restraining tube
• Energy dissipation through tension – compression
yield of steel insert, without buckling
• Reliable hysteretic response with limited slip
Starseismic.net
• Designers can reliably predict overstrength so can
de-tune member categories for design
• Prone to high residual drifts
• Expense to import proprietary systems prohibitive
Starseismic.net

Evolution towards Low Damage Design for CBF’s
(c) Asymmetric Friction Connections (AFC’s)
• Need for local innovation to reduce cost
• AFC’s Conceptualised with SHJ development
• Central cleat slides between two shim plates and
dissipates energy through friction
• Bolts slide in elongated holes in central cleat
• Ductility concentrated in connection through sliding,
protects brace, beam and column from inelastic action
Chanchi (2012)

Asymmetric Friction Connections–Concept and Configuration
Proposed AFC configurations for CBF
frames (Chanchi et. al. (2012).

Asymmetric Friction Connections–Concept and Configuration
AFC frame details from
Aurecon SCNZ Case Study
Building Design.

Asymmetric Friction Connections–Concept and Configuration
• Stable elasto-plastic
hysteresis
• Good stiffness and strength
stability after large number of
cycles
• Variety of shim materials
tested (Brass, Steel,
abrasion resistant steel)
• High hardness, abrasion
resistant steel produces
most stable response
(Chanchi et. al. (2012).

Key Design Considerations for AFC’s - Existing Theory and
Developments Between Industry and Research
(a) Slot Detailing
• Slot length “L” is determined from frame
geometry and drift
• Slot length must accommodate brace
displacement at MCE + construction
tolerance (±25mm is proposed)
• Oversizing of slotted holes is advised to
create “pinned” brace end
db
=
L = φs
× ( 2db
+ d )
H
2
+
2
( S + Δ) − B
(Chanchi et. al. (2012).

Key Design Considerations for AFC’s - Existing Theory and
Developments Between Industry and Research
(b) Joint Yield Strength
Bolt Arrangement
Dependable Sliding Force ΦFs
4/M16 128 kN
• Sliding force, F s , is function of effective
friction coefficient “μ”, number of shear
planes “n” and bolt proof load “N tf ”.
2/M16 and 2/M20 162 kN
4/M20 195 kN
2/M20 and 2/M24 238 kN
F s = μ . n . N tf
μ = 0.21 – MacRae (2005)
n=2
N tf = table 15.2.5.1 NZS 3404
Strength reduction factor Φ=0.80
• Range of connection strength achieved
with mixed bolt sizes
4/M24 282 kN
4/M20 and 2/M24 336 kN
4/M24 and 2/M20 380 kN
6/M24 423 kN
4/M24 and 4/M20 477 kN
6/M24 and 2/M20 521 kN
8/M24 565 kN

Key Design Considerations for AFC’s - Existing Theory and
Developments Between Industry and Research
(c) Capacity Design
• Overstrength factor Φ os =1.4 (Accounts for connection strength increase,
variation in bolt tightening and environmental effects).
• Brace and collector beam designed as Cat. 3 with slenderness < 120
• Column designed as Category 2 Member
• Maximum Ductility is determined considering “No Sliding” SLS seismic and ULS
wind requirement (C dT,ULS > C dT,SLS & ULS Wind).
• AFC Slot length must accommodate MCE brace displacement ±25mm
tolerance.

Key Design Considerations for AFC’s - Existing Theory and
Developments Between Industry and Research
(d) Connection Component Design
• Brace must be designed for combined axial load and bending moment from
connection eccentricity
• AFC cleats designed in accordance with HERA Report R4-142:2009 Eccentric
Cleats in Compression.
• Stiffening of cleats is likely to be require, to ensure lateral stability under
compression loading and to transfer bending moments through the connection
and into the brace

Key Design Considerations for AFC’s - Existing Theory and
Developments Between Industry and Research
(e) Connection Installation Considerations
• Provide Belleville springs between bolt heads and outside plates is recommended
- Minimises variability in bold sliding forces
- Reduces elastic strain losses
• Use hardened abrasion resistant steel shims in AFC’s
- More stable hysteresis than brass or mild steel
- Limited to interior or weather proofed applications, corrosion issues with shims in
exposed conditions
• Bolt Tensioning Procedure is Important
- Tensioning to proof load considering washers need to be flattened
- Bolts are snug tightened then tensioned a further 120°(Part Turn Method)

Key Benefits of AFC’s over other CBF Systems
• Inelastic behaviour concentrated in the brace connection – high degree of
protection to the brace, beam and column
• Non-linear response and overstrength is more reliably predicted
• Relaxation of design forces and slenderness requirements can be applied
• Significantly less severe beam and column design actions by avoiding brace
buckling
• Ability to assess and recover primary seismic frame in a reasonably timely
manner (bolt/connection inspection and bolt re-tightening/replacement)

Key Benefits of AFC’s over other CBF Systems
Post Earthquake Recovery of AFC Frame Buildings
- Building can be assessed and recovered in a relatively timely manner
- Connections require inspection to assess the extent of sliding after a significant
earthquake
- If sliding in the connection has been activated, bolts will require re-tightening are
but unlikely to require replacement
- Plan inspection port locations within architecture to allow ease of inspection and
recovery of AFC connections

Conclusions and Summary
• Reliable inelastic behaviour without brace buckling
• Reliable source of energy dissipation with stable response
• Connection is easily tuned to meet ductile demand = capacity design efficiencies
• Lower loads and improved efficiency for brace, collector beam and column
• Better performance and resilience by avoiding brace buckling
• Local and innovative NZ alternative to expensive imported solutions

Acknowledgements
• Sean Gledhill and Masoud Moghaddasi (Aurecon)
• Jose Chanchi (The University of Canterbury)
• Greg MacRae (The University of Canterbury)
• Charles Clifton (The University of Auckland)
• Hsen Han Khoo (The University of Auckland)
• Steel Construction New Zealand (SCNZ)